Coastal Environment

adapted to HTML from
lecture notes of Prof. Stephen A. Nelson Tulane University

Coastal
Processes

Waves
and
Beaches

Waves are the result of
water
oscillating in a circular motion. The water is not actually travelling
like most people seem to think. (Imagine a "wave" at a baseball game,
with
the people as water particles.)

Close to the beach, the
circular
water particle motion gets broken up as the cycling water hits the
bottom.
This causes the wave to rise up (as the water piles up on itself) and
break.

Waves rarely hit the beach
head on (at a flat angle). Most of the time, there is some small angle
between the line of the beach and the line of the waves.

As a result a Long Shore
Current forms.

Berms - Areas that
result from sand deposited on the recreactional beach in fair weather
during
periods of relatively low wave energy. During storms or periods of high
wave energy the beach is eroded.

Seacliff - During
storms the dunes may be eroded leaving a cliff scarp.

Rip Currents are areas
of strong back-current under the surface that returns
breaking
wave
water to deeper regions.

On the west coast of the
US the Pacific ocean has larger waves during the winter which results
in
erosion of the recreational beach. On the east coast the periods of
larger
waves are more related to storm activity which may be associated with
hurricanes
in the summer of fall and northeasters mainly in the winter.

Strong waves enough energy
to carry sand away from the beach after they break resulting in erosion
of the recreational beach. Most of this sand is deposited in bars just
off shore. Gentler waves lose most of their energy when they break, and
so carry sand from the off-shore bars and deposit the sand on the beach.

As a result of this, the
sizes and features of recreational beach and off-shore bars vary
significantly.

Headlands are regions
where the land thrusts out into the water.

Embayments are regions
between headlands.

Headlands tend to be
the focus of wave energy, whereas waves tend to diffuse and weaken when
in embayments. As a result of this, sediment is eroded off of
headlands,
and deposited in embayments, which eventually results in the
eventual
flattening of the coastline.

Coastal
Erosion

Wave
Erosion

Sea cliffs are a result of
wave erosion eroding away the base of a cliff face. The face
subsequently
slumps due to lack of support.

Long
Shore Currents and Human Erosion Control

Long shore currents
transfer
sand down-current. When they run into a barrier, such as a groin (a
pile
or rocks stretching into the water), the sand is caught on the upstream
side of the groin.

Littoral Drift is the same
thing as longshore current.

Jetties - Are used
to protect inlets. Jetties keep the river or inlet openings from
migrating.

-Sand builds up on upstream
jetty, and severe erosion takes place on the down-current side of the
opposing
jetty.

Barrier
Beaches

they have existed for many
thousands of years. As sea level has risen they have migrated towards
the
shore line. Currently, sea level is rising at about 30cm a century at
Long
Island.

A typical barrier beach
system:

During a storm, sand can
be overwash the island ending up in the lagoon. Repeated storms along
with
sea level rise results in the barrier beach migrating shoreward as sand
is removed from the ocean side and added in the lagoon.

Sea
Walls

During winter or storms,
sand gets carried off into an offshore sand bar (usually below the
surface
of the water).

This sand is replaced
during
the summers or low-storm periods.

When you have a sea wall
(A large wall-like structure running parallel to the beach used to stop
the waves from eroding beaches), everything in front of the sea wall
gets
eroded away during winters or storm times, and then the gentle summer
waves
have no beach slope upon which to redeposit the sand. As a result,
erosion
in front of the sea wall takes place at a much faster rate, and isn't
replaced.

Sea walls, due to the fact
that they are constantly being pounded on by waves, tend to eventually
break down, and are often very costly to repair.

Opinions
on Coastal Processes

Among environmental geologists,
there are several points of agreement with regard to coastal processes
and engineering. They are:

Coastal Erosion is a
natural
process rather than a natural hazard; erosion problems occur when
people
build structures in the coastal zone.

Any shoreline construction
causes change.

Stabilization of the
coastal
zone through engineering structures protects the property of relatively
few people at a larger general expense to the public.

Engineering structures
designed
to protect a beach may eventually destroy it.

Once constructed,
shoreline
engineering structures produce a trend in coastal development that is
difficult
if not impossible to reverse.

Storms

Hurricanes - Hurricanes
raise
up the sea level around them as they travel closer to shore. This is
referred
to as a Storm surge, as it causes a "surge" in the water level and wave
energy when the hurricane hits shore.

The wind in a hurricane in
the northern hemisphere rotates counterclockwise. Thus as the hurricane
moves northerly, as it does at higher latitudes, the winds on the east
side of the eye of the hurricane will be much faster than elsewhere
around
the eye. Likewise the winds on the west side of the eye will be much
slower
than elsewhere. See the illustration below.

Possible storm surge
patterns
around New York City and Long Island if the eye of a hurricane crosses
just west of NYC.